1. Field of the Invention
The present invention relates to a working fluid supply device of an electric discharge machine.
2. Description of the Related Art
A working fluid supply device is used in a wire electric discharge machine, which is a machine tool configured to perform machining of a workpiece based on electric discharge that occurs between a wire and the workpiece in a working fluid. The working fluid supply device serves to clean the working fluid, adjust the electrical resistance and temperature of the fluid, and supply the fluid to a working tank of the discharge machine. Working fluid supply devices of electric discharge machines can be roughly classified into two types, a cleaned-fluid storage type shown in FIG. 2 and a contaminated-fluid storage type shown in FIG. 3.
First, a working fluid supply device of the cleaned-fluid storage type will be described with reference to FIG. 2. In the description to follow, only those elements which are associated with the working fluid supply device of an electric discharge machine are illustrated, and illustration of a workpiece, wire electrode, structure for stretching the wire electrode, etc., is omitted.
Normally, electric discharge machining is performed with a workpiece immersed in a working fluid in a working tank 5. The working fluid is supplied to and stored in the working tank 5 by drawing up a cleaned fluid 10 from a cleaned-fluid tank 7 by means of a reservoir pump 1.
If electric discharge machining is performed in the working tank 5, the working fluid in the working tank is contaminated with chips produced by the machining. If the chips are trapped between the wire and the workpiece, the electric discharge machining becomes unstable. To achieve stable electric discharge machining, the cleaned fluid 10 is continuously injected from upper and lower nozzles 14 and 15 by a working fluid pump 4, whereby the space between the wire and the workpiece continues to be filled with the cleaned fluid suitable for the machining. The working fluid contaminated with the chips is returned as a contaminated fluid 9 from the working tank 5 to a contaminated fluid tank 6 through a pipe (not shown). As the contaminated fluid 9 is force-fed to a filter 8 by a filter pump 2, it is delivered as a cleaned fluid cleared of chips into the cleaned-fluid tank 7.
Since the accuracy of electric discharge machining varies depending on the temperature and electrical resistance of the working fluid, stable machining cannot be achieved by means of the simply cleaned fluid cleared of chips. Accordingly, the cleaned fluid 10 in the cleaned-fluid tank 7 is drawn up by a circulation pump 3, delivered to an ion-exchange resin 12 for adjusting the electrical resistance of the working fluid and a refrigerator 13 for temperature adjustment, and returned to the cleaned-fluid tank 7. In this way, the cleaned fluid 10 is adjusted to be suitable for electric discharge machining. The circulation pump 3 also has the function of delivering the cleaned fluid 10 directly from the cleaned-fluid tank 7 into the working tank 5 so that the working tank 5 continues to be supplied with the working fluid, in order to prevent various parts of a sealing mechanism of the tank 5 from being contaminated by adhesion of chips.
A working fluid supply device of the contaminated-fluid storage type will be described with reference to FIG. 3.
In the working fluid supply device of the cleaned-fluid storage type shown in FIG. 2, the reservoir pump 1 draws up the cleaned fluid 10 in the cleaned-fluid tank 7 and supplies it to the working tank 5. In the working fluid supply device of the contaminated-fluid storage type shown in FIG. 3, in contrast, the reservoir pump 1 draws up the contaminated fluid 9 in the contaminated fluid tank 6 and supplies it to the working tank 5.
A number of pumps are used in a working fluid supply device of a conventional wire electric discharge machine, whether of the cleaned-fluid storage type or of the contaminated-fluid storage type, as shown in FIG. 2 or 3. If the pumps are increased in number, however, they entail an increase in the size of the installation space and the number of associated parts, such as controllers, wires, etc. The increase in the number of parts in the working fluid supply device results in an increased failure rate and entails higher costs and larger space.
On the other hand, Japanese Patent Application Laid-Open No. 9-253944 discloses a working fluid supply device configured to switch, by means of an external operation, between a mode (first fluid circuit) in which a contaminated fluid 9 in a contaminated fluid tank 6 is drawn up by a single pump 16 and force-fed to a filter 8 through a first valve or a mode (second fluid circuit) in which the contaminated fluid 9 is supplied to a working tank 5 through a second valve, as shown in FIG. 4.
While the number of pumps used can be reduced with use of the working fluid supply device shown in FIG. 4, the contaminated fluid 9 drawn up from the contaminated fluid tank 6 by the pump 16 is contaminated water that is not yet filtered by the filter 8. If the fluid circuit through which the unfiltered contaminated fluid 9 passes is changed by the valves, then chips will adhere to the valves, possibly causing failure. If the contaminated fluid 9 in the contaminated fluid tank 6 is supplied directly to the working tank 5, moreover, then a workpiece will be immersed in a working fluid whose electrical resistance and temperature are not correctly adjusted due to inclusion of chips. As compared with the case of the working fluid supply device of the cleaned-fluid storage type, this is not desirable for electric discharge machining that varies in stability depending on the conditions of the working fluid.
As described above, the working fluid supply device of an electric discharge machine uses a plurality of pumps configured to supply the working fluid to a plurality of liquid circuits. However, the increase in the number of pumps used entails increased installation space, controllers, wires, etc. Thus, the greater the number of pumps used in the working fluid supply device, the higher or larger the required costs and space will be.